The invention relates to an apparatus for thermally processing a wafer at an elevated temperature, the apparatus having at least one processing position, the or each processing position being bounded at least on one side by a housing part which, during use, is at an elevated temperature.
An apparatus for processing wafers in which the processing position is bounded by housing parts at an elevated temperature has been described in, for example, Dutch patent 1003538 in applicant""s name. The processing of a wafer can comprise, for instance, deposition or etching of material, or annealing or implantation of doping. Such wafer processing takes place at a processing temperature which is usually above room temperature, and, in some processing operations, may well involve a value in excess of 1000xc2x0 C. In this known apparatus, the processing position is formed by a process chamber having such dimensions that it encloses the wafer very tightly. For this reason, the wafer should be positioned accurately in the process chamber during loading and during processing, so that damage to the wafer is prevented. Damage can occur during loading, when the wafer, as a result of an incorrect positioning, contacts a wall of the process chamber. Moreover, the wafer and/or the apparatus can be damaged as a consequence of an incorrect positioning of the wafer during processing, e.g. because the wafer drops from wafer support means on which the wafer rests during processing. Usually, during processing, a wafer is rotated around its center to increase the uniformity of the processing. This rotation must be terminated after processing, so that the wafer can be taken out of the process chamber by wafer transport means. If the wafer still rotates, the contact between the wafer and the wafer transport means can also lead to damage to the wafer and the wafer transport means. In the apparatus known from the Dutch publication, the wafer is supported during processing by a gas bearing, so that there is no mechanical contact.
From the above it will be apparent that there is a need to measure the position and the possible rotation of the wafer prior to and/or during the processing operation.
The object of the invention is to provide an apparatus for processing a wafer of the type described in the preamble, in which contactless measurement of the position of the wafer in the processing position can take place.
To this end, the invention provides an apparatus of the type described in the preamble, which is characterized in that the apparatus is provided with measuring means for determining the position of a wafer in the processing position prior to and/or during processing, the measuring means being provided with at least one signal processor and at least one signal conductor, the or each signal processor being located at a distance from the processing position in an area of lower temperature, the signal conductor extending through or over the housing part being at an increased temperature and extending from a measuring point in or adjacent the processing position to the signal processor for transmitting to the signal processor contactlessly obtained measuring signals originating from the measuring point, while the at least one signal conductor, at least the part extending through or over the housing part being at an increased temperature, is heat resistant.
As the measuring signals are contactlessly obtained, the wafer can be situated contactlessly in the processing position during processing, for instance in that it is supported by a gas bearing. However, this is not requisite, the wafer can also rest on a support element. To determine the position of the wafer, however, mechanical contact with the wafer is not necessary.
As the signal conductors are heat resistant, they can extend through a housing part being at an elevated temperature, so that the signal, obtained contactlessly, can be conducted from the area being at an elevated temperature to an area where a lower temperature prevails, where the signal processor is safely arranged.
According to a further elaboration of the invention, housing parts being at an elevated temperature can be provided on opposite sides of the or each wafer processing position. Thus, a good heat transfer to the wafer can be realized.
Additionally, according to a further elaboration of the invention, signal conductors can extend through or over both housing parts, while signal conductors, at least the parts extending through or over the housing parts being at an elevated temperature, are heat resistant.
The signal conductors extending through or over a first housing part can, for instance, be connected to a signal source, and the signal conductors extending through or over a second housing part can be connected to a signal processor.
According to a further elaboration of the invention, the or each signal processor can comprise an optical sensor, the or each signal conductor, at least the part extending through or over a housing part being at an elevated temperature, being manufactured from light conducting, heat resistant material. The light conducting, heat resistant material can be, for instance, quartz glass.
In this further elaboration, the signal source is preferably a light source. Outside the housing parts being at an elevated temperature, the signal conductors can be designed as glass fiber cable.
The signal conductors conduct the light originating from the light source to the measuring points, which are located preferably near the intended position of the wafer edge. Insofar as the edge of the wafer does not completely cover the measuring point, light is passed wholly or partly and fed via signal conductors to the at least one optical sensor. On the basis of the electric signal produced by the optical sensors, the position of the wafer can be determined. Because the optical sensors and the light source are situated at a distance from the processing position, these will not, during processing, come into contact with, for instance, a processing gas or a processing temperature which could damage the sensors or the light source.
A wafer which has been heated, for instance by the processing temperature of a wafer processing operation, also emits light itself. This can complicate the optical detection of the wafer edge which is lit up for detection in the process chamber. To resolve this problem, the light source can for instance emit modulated light. What is meant by modulated light is light whose intensity varies periodically. This modulated light, together with the unmodulated light emitted by the heated wafer, is detected by the optical sensors and converted into a sensor signal. The amount of modulated light in the sensor signal can then be calculated through simple signal processing.
In an alternative embodiment, the or each signal processor can comprise a pneumatic pressure sensor or acoustic sensor, the or each signal conductor, at least the part extending through a housing part being at an elevated temperature, comprising a fluid passing channel in said housing part, which channel is in fluid communication with the pneumatic pressure sensor or acoustic sensor. Further, the signal source can comprise a pneumatic pressure source or a sound source. The pneumatic pressure source can be, for instance, an inert gas source, feeding gas through a fluid passing channel to the processing position. The pneumatic pressure source can also be formed by the gas being fed to the processing position for processing the wafer. The pneumatic pressure sensor can, for instance, measure the gas pressure in the above-mentioned gas feeding, fluid passing channel. If the wafer partly covers the exit of the gas feeding fluid passing channel, the pressure in that channel will increase, which is an indication for the position of the wafer. However, it is also possible that in an opposite housing part a second fluid passing channel is present, extending in alignment with the first channel, in which the gas pressure is measured by means of the pneumatic pressure sensor. If the wafer partly covers this second channel, the gas pressure in it decreases, which is also an indication for the position of the wafer.
When use is made of a sound source and an acoustic sensor, the amount of sound is measured which is passed or reflected through the fluid channel which may or may not be partly covered by the wafer.
It will be clear that also in this alternative elaboration a contactless measurement takes place and that the signal of the measurement is conducted by the heat resistant signal conductor in the form of a fluid channel from the processing position to a signal processor which is in a friendlier environment.
xe2x80x98Elevated temperaturexe2x80x99 is herein to be understood to mean a temperature of over 200xc2x0 C., more in particular over 400xc2x0 C.
In some apparatuses which are provided with two housing parts being at an elevated temperature, these housing parts are arranged so that they can be moved away from and towards each other. Such an embodiment enables the positioning of the wafer in the processing position, while, in operation, a substantially fully closed process chamber can be used. According to a further elaboration of the invention, it is particularly favorable when the apparatus is provided with a control comprising or being connected to the or each signal processor, the control being arranged to operate the movement away from and toward each other of the first and second housing parts, while the control can solely give a command for the movement toward each other of the housing parts when the signal processor determines that the wafer is in the correct position or when the signal processor determines that there is no wafer present in the processing position. This prevents the housing parts from moving towards each other while the wafer is not yet in the correct position. If the latter should happen, it could have disastrous consequences, necessitating, for instance, a revision of the apparatus.
The control is preferably arranged to control, on the basis of the measuring signals, transport means for correcting the position of the wafer, so that the wafer is placed in the desired position. With the aid of the control, loading the wafer into the apparatus can be carried out automatically and accurately.
Possibly, in a housing part as mentioned, gas feed channels can be arranged for forming a gas bearing in the processing position. With such a gas bearing, there is the danger that the wafer displaces laterally in the processing position, so that the wafer loses its desired position. To prevent this, an apparatus of such design can be provided with a control comprising or being connected to the or each signal processor, the control being arranged for regulating the gas flow in the gas feed channels, the gas flow being regulated depending on measuring signals determined by the signal processor, in such a manner that the wafer maintains the correct position during processing.
Thus, during processing, the correct position can be maintained without the wafer contacting any mechanical part.
Furthermore, the gas flows for forming the gas bearing can be such, that the wafer rotates during processing, while the control is arranged for determining, depending on the measuring signals processed by the signal processor, the speed of rotation of the wafer, the control being arranged for regulating the gas flow for obtaining the desired rotational speed.
In order to be able to accurately determine the position and the rotation, according to a further elaboration of the invention, the apparatus is provided with at least three measuring points. These measuring points are preferably situated at the intended position of the wafer edge in the processing position. Preferably, the or each signal processor is arranged to determine the extent to which the measuring point is covered by the wafer being in the processing position.
It is noted that from WO98/57359 an apparatus for processing a wafer at an elevated temperature is known, in which the wafer, during processing, is supported by a wafer support pedestal, in which pairs of electrodes are incorporated for determining, by means of a capacitance measurement, the presence and absence as well as the position of the wafer. The value of the capacitance of a pair of electrodes is influenced by the presence of the wafer and the position of the wafer in the processing position.
A drawback of the known apparatus is that the wafer makes contact with or is located at a small distance from the pairs of electrodes. Not nearly in all thermal processing operations is such contact or exposure to electrode material allowable or desirable. The measuring means known from the above mentioned international patent application which are based on mechanical contact with, or at least exposure to pairs of electrodes are therefore not suitable for determining the position of the wafer in the wafer processing furnace from the Dutch publication.
Moreover, the international publication does not describe how the signals of the pairs of electrodes are conducted to the signal processor. Especially when the processing position is bounded by housing parts being at a very strongly elevated temperature, for instance a temperature of over 600xc2x0 C., this signal conduction is a problem. For this problem, the present invention also offers a solution.